This invention relates, in general, to electric power generating systems; and, in particular, this invention relates to electric power generating stations of the type utilizing power system stabilizer devices for modifying electric generator field control signals and further relates to a means for attenuating the output of the power system stabilizer at shaft natural torsional frequencies.
In general, a power station for generating electrical power comprises a turbine, an electrical generator and an exciter which may all be interconnected by a rotating shaft. Several or more power stations may be electrically interconnected to form a power grid. Power grids may themselves be electrically interconnected. It has been found that two or more power stations electrically interconnected may be susceptible to local mode oscillations in the range of from 1 to 2 Hertz due to the effect of one large rotating mass from one power station interacting with another large rotating mass from another power station. Additionally, when one power grid is tied to a second power grid, so-called inter-tie mode oscillation may occur in the range of 0.3 to 0.5 Hertz.
The foregoing local mode and inter-tie mode oscillations may be effectively damped by utilizing a power system stabilizer. The power system stabilizer inputs a phase adjusted speed, power or frequency signal into the generator field control to provide damping for local or inter-tie mode oscillations. Power system stabilizers are documented in the literature as, for example, "System design Considerations and Operating Experience with High Performance Thyristor Excitation Systems" by Farmer, Crenshaw, Schulz and Temoshok; Volume 33, Proceedings of the American Power Conference, 1971; and, "Design of a Power System Stabilizer Sensing Frequency Deviation" by Keay and South; Volume PAS-90, No. 2, IEEE Transactions on Power Apparatus and Systems.
While power system stabilizers have been shown effective in damping local mode and inter-tie mode oscillations, in some instances they have been shown to have a destabilizing effect on torsional modes of oscillation. This has been documented in "Static Exciter Stabilizing Signals on Large Generators--Mechanical Problems" by Watson and Coultes; Volume PAS-92, pages 204-211, IEEE Transactions on Power Apparatus and Systems. Torsional mode oscillations refer to each rotating mass on a common shaft and the interaction among the rotating masses. Normally, on a common shaft of n rotating masses, there will be n-1 modes or torsional oscillation in the subsynchronous range. These torsional oscillations may exhibit negative damping characteristics in the presence of a power system stabilizer input, and if left to grow could cause the shaft to break. In this case, it would be better to have deleted the input signal from the power system stabilizer.
One solution found in the literature relative to damping torsional oscillations suggests that a shaft speed pickup might be selectively placed at a node occurring during the first mode of torsional oscillation. This becomes a complex task, if the node physically occurs within one of the rotating masses; and further, the approach requires a complex analysis for each machine to which it is to be applied. Clearly, a universal and less complex solution is required for the electrical power generating industry with the caveat that should the solution fail, ample safeguards be present to deactivate the power system stabilizer.